Summer offers unique opportunities for the wearing of sweaters. Heat is only one parameter in the equation, along with activity level, prevailing wind, humidity, and the possibility of day-into-night dressing on holiday. A few types of sweater are particularly suited for the warm season:
Polo sweaters, whose ribbed cuffs and hem present a sleek counterpart to generally looser summerwear—most notably when rendered in a nicer cotton with a bit of sheen.
T-shirt sweaters, preferably in interesting slubby textures, with airy open knits and boat necks to combat solar gain.
Long-sleeved linen sweaters, the kind you pull on poolside or push up to your elbows, accepting that they'll be stretched out and worn by fall. Fast-fashion brands like Zara and H&M are great for this sort of thing.